Final Warning is a Lightfall-exclusive exotic and is acquired once you have unlocked all Strand subclass fragments! You must have already completed any version of the Lightfall campaign as well as unlocked all Strand fragments to have access to this quest.
Final Warning is a KINETIC exotic sidearm that is STRAND damage.
All at Once
Holding down the trigger marks targets within range and loads multiple bullets, which fire in a burst with increased stability on trigger release. Hitting a marked target with a fully charged burst will unravel them.
You must be 1750+ Light Level and have ALL Strand fragments unlocked
You must own the Lightfall DLC and have already completed the campaign